1. Field of the Invention
The present invention relates to a method of filling large-capacity storage silos with a fluidizable material, and to an arrangement for filling the same.
2. Description of the Related Art
When filling large-capacity silos, in particular with fine-grain, easily fluidizable bulk materials with particle sizes of less than 200 μm, it is observed that air flows and dead spaces which give rise to occurrences of separation in accordance with particle size are established above the silo filling. The fine material is preferably deposited in the outer silo sections, in the vicinity of the wall, or in the dead spaces of the air flow, and the less fine material is preferably deposited in the centre of the silo. The phenomenon of air-flow separation is established, in particular, in storage silos for aluminium oxide, the silo having diameters of over 30 m and in which, with moisture content <1%, the material is very easily fluid and fluidizable and the particle size ranges from approximately 1 μm to 200 μm. For example a fraction of approximately 5-10% of fine material with particle sizes <45 μm is present here on average. Within the silo, the fine material is distributed with a decreasing concentration from the outer section at the wall to the inner section in the centre. In unfavourable cases, it may be that individual outer silo sections have an accumulation of up to 70% of fine material. If the separated material is removed, this results in considerable problems in subsequent further processing for the production of aluminium.
In order to keep the separation effects to a low level, a series of methods has been developed up until now. Gisle G. Enstad's publication in POSTEC Newsletter No. 16, December 1997, pages 27-30 proposes a central filling pipe which runs from the silo top to the silo base and allows filling to take place from bottom to top through a plurality of opening flaps. DE 600 14050 T2 uses a plurality of distributor pipes which run along the wall in the interior of the silo. The material is largely vented, prior to entering into the silo, via a venting box and, if appropriate, a material distributor. The silo or the large-capacity container has inlets for ambient air and outlets for the removal of air. To this extent, this means can at best, if at all, counteract air-flow separation, but cannot prevent it, because the material only displaces the air in the silo during filling and dead spaces are created for the air flow. Secondly, it is also the case that the pipes constitute significant installations in the silo which, although more robust than in the first-mentioned publication, are subjected to large material forces and can therefore fracture and, during emptying, tend to block up as a result of the material compacting over time. The abovementioned DE document attempts to take account of the failure of distributor pipes by way of branch lines or bypass lines, but it is also possible for blocking to occur in any bypass line.
It is an object of the present invention completely to avoid the above problems, in particular the dead spaces for the air flow in the outer silo sections, in the vicinity of the wall, within the large-capacity silo, in order that separation can be reliably avoided. The object of the invention is also to provide a corresponding arrangement which is straightforward to construct and to operate and is cost-effective to produce. These objects are achieved according to the invention by the method and by the arrangement herein.
At the same time, the silo is filled in a controlled manner such that the fine material is deposited mostly in the inner sections, while the coarse material is deposited mostly in the outer sections, this counteracting the natural distribution on account of the air flow in the silo. The method of distribution can be influenced by the specific suction extraction of air in the silo according to the invention. The invention forms an arrangement which is not provided in the material and is not subjected to any silo forces and does not result in any disturbances during silo operation.
The arrangement serves for filling large-capacity silos with materials which are fluidizable and tend to show signs of separation as a result of the air flows in silos. The invention will be explained in more detail hereinbelow with reference to drawings which are intended to give a better understanding of the invention, although the latter is not restricted thereto.